The present invention relates to water processable photoresist compositions, their uses and methods to prepare such photoresist compositions.
Photoresists are compositions that are capable of transferring an image onto a substrate, such as a printed circuit board, silicon wafer, or a lithographic plate. After a layer of a photoresist is deposited on a surface as a thin film, the film is selectively exposed, typically through a photomask, to a source of activating radiation, such as ultraviolet light or an electron beam. The photomask has areas that are opaque and other areas that are transparent to the activating energy. The pattern on the photomask formed by opaque and transparent areas defines the pattern, such as a circuit, to be transferred to the substrate.
Once the substrate has been exposed selectively to activating radiation in the desired pattern, the film layer must be modified, or developed. Development of the image transferred to the substrate requires the selective removal of the photoresist in accordance with the pattern. Photoresists can be characterized as positive active or negative active, alternatively referred to as producing a positive tone or negative tone, respectively. Exposed portions of a negative-active photoresist become less soluble in a developing solution than unexposed areas as the result of a photochemical reaction. This difference in solubility allows for the selective removal of the photoresist in the unexposed areas and the transfer of the photomask image to the substrate. In the case of positive-acting photoresists, exposure to activating radiation will cause the coating to become more soluble in a developing solution than the unexposed areas. The imaged photoresist may act as a protective coating for subtractively etching a metal substrate, known as print-and-etch, or allow the exposed bare metal surface to be further built up in thickness by electroplating methods, known as print-and-plate. The resist may then be selectively stripped. For print-and-etch processes, the exposed metal on the substrate may be etched to form the desired pattern or circuit on the substrate surface.
Photoresist compositions include a photoacid generator (PAG), which is selectively activated by exposure to the activating radiation to produce an acid-catalyzed transformation in the resist film in the regions transparent to the photomask. Sensitizers and other components also may be employed to optimize the properties of a given photoresist.
The photoresists used by the semiconductor industry are typically cast from organic solvents, and in many cases, require organic solvents or cosolvents as developing solutions. Organic solvents present a number of disadvantages, including the potential for unintended release to the environment, operator safety hazards, and negative economic impacts. Thus there is a present need to develop water-processable photoresist compositions. The present invention fulfills this and other needs.
The present invention relates to photoresists that can be cast from and developed with aqueous media, precluding the need for significant use of organic solvents. The water-processable, positive-tone photoresist compositions of the present invention comprise a water-processable photoacid generator, water, and a water-soluble polymer, said polymer containing a heat-labile functional group that, when activated by heat, renders said polymer insoluble in water or aqueous base; and said polymer also contains an acid-labile functional group that, when activated by acid produced by action of the photoacid generator, renders said heat-treated polymer soluble in water or aqueous base.
The present invention also includes a water-processable positive-tone photoresist composition comprising:
(a) a polymer comprising at least one salt of a carboxylic acid and at least one acid labile group and wherein said polymer undergoes decarboxylation upon heat treatment and wherein said polymer undergoes cleavage upon acid treatment;
(b) a water-soluble photoacid generator; and
(c) water or an aqueous base;
the composition being characterized in that a dry film of the composition can be prepared; wherein the film is soluble in water or an aqueous base prior to the heat treatment, and insoluble in water or aqueous base after the heat treatment of said polymer; and wherein exposure of said film to radiation renders said film soluble in water or said aqueous base.
Further provided are met hods for producing positive-tone photoresist compositions comprising the steps of:
(a) applying to a substrate a solution comprising a water-soluble polymer capable of being converted to a water-insoluble form of the polymer upon heating and capable of being restored to a water-soluble form of the polymer upon irradiation, wherein the polymer contains a heat-labile functional group and an acid-labile functional group; a water-processable photoacid generator; and water or an aqueous base; to produce a coated substrate;
b) heating the coated substrate to remove the water from said substrate to form a film on the substrate, and to convert the water-soluble polymer to a water-insoluble form of polymer;
(c) selectively exposing said film to irradiation to generate acid from the water-processable photoacid generator, such that the generated acid restores the water-insoluble polymer to a water-soluble form of the polymer; and
(d) developing the exposed from with an aqueous developer to produce the positive-tone photoresist.
Also provided by the present invention are water-processable positive-tone photoresist compositions made by the methods or processes described.